<meta charset="utf-8" lang="kotlin">

# Lint Check Unit Testing

Lint has a dedicated testing library for lint checks. To use it,
add this dependency to your lint check Gradle project:

```
testImplementation "com.android.tools.lint:lint-tests:$lintVersion"
```

This lends itself nicely to test-driven development. When we get bug
reports of a false positive, we typically start by adding the text for
the repro case, ensure that the test is failing, and then work on the
bug fix (often setting breakpoints and debugging through the unit test)
until it passes.

## Creating a Unit Test

Here's a sample lint unit test for a simple, sample lint check which
just issues warnings whenever it sees the word “lint” mentioned
in a string:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
package com.example.lint.checks

import com.android.tools.lint.checks.infrastructure.TestFiles.java
import com.android.tools.lint.checks.infrastructure.TestLintTask.lint
import org.junit.Test

class SampleCodeDetectorTest {
    @Test
    fun testBasic() {
        lint().files(
            java(
                """
                package test.pkg;
                public class TestClass1 {
                    // In a comment, mentioning "lint" has no effect
                    private static String s1 = "Ignore non-word usages: linting";
                    private static String s2 = "Let's say it: lint";
                }
                """
            ).indented()
        )
        .issues(SampleCodeDetector.ISSUE)
        .run()
        .expect(
            """
            src/test/pkg/TestClass1.java:5: Warning: This code mentions lint: Congratulations [SampleId]
                private static String s2 = "Let's say it: lint";
                                           ∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼
            0 errors, 1 warnings
            """
        )
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Lint's testing API is a “fluent API”; you chain method calls together,
and the return objects determine what is allowed next.

Notice how we construct a test object here on line 10 with the `lint()`
call. This is a “lint test task”, which has a number of setup methods
on it (such as the set of source files we want to analyze), the issues
it should consider, etc.

Then, on line 23, the `run()` method. This runs the lint unit test, and
then it returns a result object. On the result object we have a number
of methods to verify that the test succeeded. For a test making sure we
don't have false positives, you can just call `expectClean()`. But the
most common operation is to call `expect(output)`.

!!! Tip
   Notice how we're including the whole text output here; including not
   just the error message and line number, but lint's output of the
   relevant line and the error range (using ~~~~ characters).

   This is the recommended practice for lint checks. It may be tempting
   to avoid “duplication” of repeating error messages in the tests
   (“DRY”), so some developers have written tests where they just
   assert that a given test has say “2 warnings”. But this isn't
   testing that the error range is exactly what you expect (which
   matters a lot when users are seeing the lint check from the IDE,
   since that's the underlined region), and it could also continue to
   pass even if the errors flagged are no longer what you intended.

   Finally, even if the location is correct today, it may not be
   correct tomorrow. Several times in the past, some unit tests in
   lint's built-in checks have started failing after an update to the
   Kotlin compiler because of some changes to the AST which required
   tweaks here and there.

## Computing the Expected Output

You may wonder how we knew what to paste into our `expect` call
to begin with.

We didn't. When you write a test, simply start with
`expect("")`, and run the test. It will fail. You can now
copy the actual output into the `expect` call as the expected
output, provided of course that it's correct!

## Test Files

On line 11, we construct a Java test file. We call `java(...)` and pass
in the source file contents. This constructs a `TestFile`, and there
are a number of different types of test source files, such as for
Kotlin files, manifest files, icons, property files, and so on.

Using test file descriptors like this to **describe** an input file has
a number of advantages over the traditional approach of checking in
test files as sources:

* Everything is kept together, so it's easier to look at a test and see
  what it analyzes and what the expected results are. This is
  particularly important for complex lint checks which test a lot of
  scenarios. As of this writing, `ApiDetectorTest` has 157 individual
  unit tests.

  ![Multiple test files shown inline](nested-test-files.png)

* Lint can provide a DSL to construct test files easily. For example,
  `projectProperties().compileSdk(17)` and
  `manifest().minSdk(5).targetSdk(17)` construct a `project.properties`
  and an `AndroidManifest.xml` file with the correct contents to
  specify for example the right <uses-sdk> element setting up the
  `minSdkVersion` and `targetSdkVersion`.

   For icons, we can construct bitmaps like this:
```
        image("res/mipmap-hdpi/my_launcher2_round.png", 50, 50)
           .fillOval(0, 0, 50, 50, 0xFFFFFFFF)
           .text(5, 5, "x", 0xFFFFFFFF))
```

* Similarly, when we construct `java()` or `kotlin()` test sources, we
  don't have to name the files, because lint will analyze the source
  code and figure out what the class file should be named and where to
  place it.

* We can easily “parameterize” our test files. For example, if you want
  to run your unit test against a 100K json file, you can construct it
  programmatically; you don't have to check one in. As another example
  you can programmatically create a number of repetitive scenarios.

* Since test sources often (deliberately!) have errors in them (which
  is relevant when lint is unning on the fly inside the IDE editor),
  this sometimes causes problems with the tooling; for example, some
  code review tools will flag “disallowed” constructs or things like
  tabs or trailing spaces, which may be deliberate in a lint unit test.

* You can test running in single-file mode, which is how lint is run
  on the fly in the editor.

* Lint originally checked in test sources as individual files.
  Unfortunately over time, source files ended up getting reused by
  multiple tests. And that made it harder to make changes, or figure
  out whether test sources are still in use, and so on.

* Last but not least, because all the test construction methods
  specify the correct mime type for their string parameters, IntelliJ
  will actually syntax highlight the test source declarations! Here's
  how this looks:

  ![Screenshot of nested highlighting](nested-syntax-highlighting.png)

* Finally, but most importantly, with the descriptors of your test
  scenarios, lint can re-run your tests under a number of different
  scenarios, including modifying your source files and project layout.
  This concept is documented in more detail in the [test
  modes](test-modes.md.html) chapter.

## Trimming indents?

Notice how in the above Kotlin unit tests we used raw strings, **and**
we indented the sources to be flush with the opening `"""` string
delimiter.

You might be tempted to call `.trimIndent()` on the raw string.
However, doing that would break the above nested syntax highlighting
method (or at least it used to). Therefore, instead, call `.indented()`
on the test file itself, not the string, as shown on line 20.

Note that we don't need to do anything with the `expect` call; lint
will automatically call `trimIndent()` on the string passed in to it.

## Dollars in Raw Strings

Kotlin requires that raw strings have to escape the dollar ($)
character. That's normally not a problem, but for some source files, it
makes the source code look **really** messy and unreadable.

For that reason, lint will actually convert $ into ＄ (a unicode wide
dollar sign). Lint lets you use this character in test sources, and it
always converts the test output to use it (though it will convert in
the opposite direction when creating the test sources on disk).

## Quickfixes

If your lint check registers quickfixes with the reported incidents,
it's trivial to test these as well.

For example, for a lint check result which flags two incidents, with a
single quickfix, the unit test looks like this:

```
lint().files(...)
    .run()
    .expect(expected)
    .expectFixDiffs(
        ""
        + "Fix for res/layout/textsize.xml line 10: Replace with sp:\n"
        + "@@ -11 +11\n"
        + "-         android:textSize=\"14dp\" />\n"
        + "+         android:textSize=\"14sp\" />\n"
        + "Fix for res/layout/textsize.xml line 15: Replace with sp:\n"
        + "@@ -16 +16\n"
        + "-         android:textSize=\"14dip\" />\n"
        + "+         android:textSize=\"14sp\" />\n");
```

The `expectFixDiffs` method will iterate over all the incidents it
found, and in succession, apply the fix, diff the two sources, and
append this diff along with the fix message into the log.

When there are multiple fixes offered for a single incident, it will
iterate through all of these too:

```
lint().files(...)
    .run()
    .expect(expected)
    .expectFixDiffs(
        + "Fix for res/layout/autofill.xml line 7: Set autofillHints:\n"
        + "@@ -12 +12\n"
        + "          android:layout_width=\"match_parent\"\n"
        + "          android:layout_height=\"wrap_content\"\n"
        + "+         android:autofillHints=\"|\"\n"
        + "          android:hint=\"hint\"\n"
        + "          android:inputType=\"password\" >\n"
        + "Fix for res/layout/autofill.xml line 7: Set importantForAutofill=\"no\":\n"
        + "@@ -13 +13\n"
        + "          android:layout_height=\"wrap_content\"\n"
        + "          android:hint=\"hint\"\n"
        + "+         android:importantForAutofill=\"no\"\n"
        + "          android:inputType=\"password\" >\n"
        + "  \n");
```

## Library Dependencies and Stubs

Let's say you're writing a lint check for something like the Android
Jetpack library's `RecyclerView` widget.

In this case, it's highly likely that your unit test will reference
`RecyclerView`. But how does lint know what `RecyclerView` is? If it
doesn't, type resolve won't work, and as a result the detector won't.

You could make your test “depend” on the `RecyclerView`. This is
possible, using the `LibraryReferenceTestFile`, but is not recommended.

Instead, the recommended approach is to just use “stubs”; create
skeleton classes which represent only the **signatures** of the
library, and in particular, only the subset that your lint check cares
about.

For example, for lint's own `RecyclerView` test, the unit test declares
a field holding the recycler view stub:

```
private val recyclerViewStub = java(
    """
    package android.support.v7.widget;

    import android.content.Context;
    import android.util.AttributeSet;
    import android.view.View;
    import java.util.List;

    // Just a stub for lint unit tests
    public class RecyclerView extends View {
        public RecyclerView(Context context, AttributeSet attrs) {
            super(context, attrs);
        }

        public abstract static class ViewHolder {
            public ViewHolder(View itemView) {
            }
        }

        public abstract static class Adapter<VH extends ViewHolder> {
            public abstract void onBindViewHolder(VH holder, int position);
            public void onBindViewHolder(VH holder, int position, List<Object> payloads) {
            }
            public void notifyDataSetChanged() { }
        }
    }
    """
).indented()
```

And now, all the other unit tests simply include `recyclerViewStub`
as one of the test files. For a larger example, see
[this test](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/SliceDetectorTest.kt).

!!! Tip
   In recent versions of lint, the unit testing library will do some
   basic checking to make sure that important symbols *do* resolve
   correctly. It doesn't check everything (since it's common for unit
   tests to contain snippets from copy paste that aren't relevant to
   the test), but it does check all classes and methods referenced via
   import statements, and any calls or references in the test files
   that match any of the names returned from
   `getApplicableMethodNames()` or `getApplicableReferenceNames()`
   respectively.

Here's an example of a test failure for an unresolved import:

```text
java.lang.IllegalStateException:
app/src/com/example/MyDiffUtilCallbackJava.java:4: Error:
Couldn't resolve this import [LintError]
import androidx.recyclerview.widget.DiffUtil;
       -------------------------------------

This usually means that the unit test needs to declare a stub file or
placeholder with the expected signature such that type resolving works.

If this import is immaterial to the test, either delete it, or mark
this unit test as allowing resolution errors by setting
`allowCompilationErrors()`.

(This check only enforces import references, not all references, so if
it doesn't matter to the detector, you can just remove the import but
leave references to the class in the code.)
```

## Binary and Compiled Source Files

If you need to use binaries in your unit tests, there are two options:

 1. base64gzip
 2. API stubs

If you want to analyze bytecode of method bodies, you'll need to use
the first option.

The first type requires you to actually compile your test file into a
set of .class files, and check those in as a gzip-compressed, base64
encoded string. Lint has utilities for this; see the next section.

The second option is using API stubs. For simple stub files (where you
only need to provide APIs you'll call as binaries, but not code), lint
can produce the corresponding bytecode on the fly, so you don't need
to pre-create binary contents of the class. This is particularly
helpful when you just want to create stubs for a library your lint
check is targeting and you want to make sure the detector is seeing
the same types of elements as it will when analyzing real code outside
of tests (since there is a difference between resolving into APIs from
source and form binaries; when you're analyzing calls into source, you
can access for example method bodies, and this isn't available via
UAST from byte code.)

These test files also let you specify an artifact name instead of a
jar path, and lint will use this to place the jar in a special place
such that it recognizes it (via `JavaEvaluator.findOwnerLibrary`) as
belonging to this library.

Here's an example of how you can create one of these binary stub
files:

```
fun testIdentityEqualsOkay() {
    lint().files(
        kotlin(
            "/*test contents here *using* some recycler view APIs*/"
        ).indented(),
        mavenLibrary(
            "androidx.recyclerview:recyclerview:1.0.0",
            java(
                """
                    package androidx.recyclerview.widget;
                    public class DiffUtil {
                        public abstract static class ItemCallback<T> {
                            public abstract boolean areItemsTheSame(T oldItem, T newItem);
                            public abstract boolean areContentsTheSame(T oldItem, T newItem);
                        }
                    }
                    """
            ).indented()
        )
    ).run().expect(
```

## Base64-encoded gzipped byte code

Here's an example from a lint check which tries to recognize usage of
Cordova in the bytecode:

```
fun testVulnerableCordovaVersionInClasses() {
    lint().files(
        base64gzip(
            "bin/classes/org/apache/cordova/Device.class",
            "" +
                "yv66vgAAADIAFAoABQAPCAAQCQAEABEHABIHABMBAA5jb3Jkb3ZhVmVyc2lv" +
                "bgEAEkxqYXZhL2xhbmcvU3RyaW5nOwEABjxpbml0PgEAAygpVgEABENvZGUB" +
                "AA9MaW5lTnVtYmVyVGFibGUBAAg8Y2xpbml0PgEAClNvdXJjZUZpbGUBAAtE" +
                "ZXZpY2UuamF2YQwACAAJAQAFMi43LjAMAAYABwEAGW9yZy9hcGFjaGUvY29y" +
                "ZG92YS9EZXZpY2UBABBqYXZhL2xhbmcvT2JqZWN0ACEABAAFAAAAAQAJAAYA" +
                "BwAAAAIAAQAIAAkAAQAKAAAAHQABAAEAAAAFKrcAAbEAAAABAAsAAAAGAAEA" +
                "AAAEAAgADAAJAAEACgAAAB4AAQAAAAAABhICswADsQAAAAEACwAAAAYAAQAA" +
                "AAUAAQANAAAAAgAO"
        )
    ).run().expect(
```

Here, “base64gzip” means that the file is gzipped and then base64
encoded.

If you want to compute the base64gzip string for a given file, a simple
way to do it is to add this statement at the beginning of your test:

```
assertEquals("", TestFiles.toBase64gzip(File("/tmp/mybinary.bin")))
```

The test will fail, and now you have your output to copy/paste into the
test.

However, if you're writing byte-code based tests, don't just hard code
in the .class file or .jar file contents like this. Lint's own unit
tests did that, and it's hard to later reconstruct what the byte code
was later if you need to make changes or extend it to other bytecode
formats.

Instead, use the new `compiled` or `bytecode` test files. The key here
is that they automate a bit of the above process: the test file
provides a source test file, as well as a set of corresponding binary
files (since a single source file can create multiple class files, and
for Kotlin, some META-INF data).

Here's an example of a lint test which is using `bytecode(...)` to
describe binary files:
[](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/client/api/JarFileIssueRegistryTest.kt?q=testNewerLintBroken)

Initially, you just specify the sources, and when no binary data
has been provided, lint will instead attempt to compile the sources
and emit the full test file registration.

This isn't just a convenience; lint's test infrastructure also uses
this to test some additional scenarios (for example, in a multi-module
project it will only provide the binaries, not the sources, for
upstream modules.)

## My Detector Isn't Invoked From a Test!

One common question we hear is

> My Detector works fine when I run it in the IDE or from Gradle, but
> from my unit test, my detector is never called! Why?

This is almost always because the test sources are referring to some
library or dependency which isn't on the class path. See the “Library
Dependencies and Stubs” section above, as well as the [frequently asked
questions](faq.md.html).

## Language Level

Lint will analyze Java and Kotlin test files using its own default
language levels. If you need a higher (or lower) language level in order
to test a particular scenario, you can use the `kotlinLanguageLevel`
and `javaLanguageLevel` setter methods on the lint test configuration.
Here's an example of a unit test setup for Java records:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin
    lint()
      .files(
        java("""
            record Person(String name, int age) {
            }
            """)
          .indented()
      )
      .javaLanguageLevel("17")
      .run()
      .expect(...)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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